1. Technical Field
The present disclosure relates to a method for controlling the temperature of a molten metal of an alloy.
2. Description of Related Art
A rare-earth magnet made of a rare-earth element, such as a lanthanoid, is also called a permanent magnet, and is used in the motors of hard disks and MRI apparatuses, as well as in the driving motors of hybrid electric vehicles, electric vehicles, etc.
Examples of rare-earth magnets include common sintered magnets of which the scale of crystal grains (main phase) constituting the structure is about 3 to 5 μm and nanocrystal magnets of which the crystal grains are refined to a nanoscale of about 50 nm to 300 nm. Among others, nanocrystal magnets in which the addition of expensive heavy rare-earth elements can be reduced (omitted) while the crystal grains can be refined are currently gaining attention.
To briefly explain a rare-earth magnet manufacturing method: for example, a molten metal of an alloy (e.g., Nd—Fe—B-based molten metal) that is a material for a rare-earth magnet is prepared inside a crucible having a nozzle at the bottom, and the molten metal is discharged downward from the nozzle and fed onto a melt-quenching rotating roll. The molten metal of the alloy having been fed onto the rotating roll is rapidly solidified by the rotating roll and turns into a quenched ribbon (quenched thin strip), and is jetted in a direction tangential to a point in the rotating roll to which the molten metal has been dripped. The quenched ribbon is ground into a desired size to obtain powder for a magnet, and this powder is sintered while being pressure-formed to manufacture a sintered body.
One of the factors determining the quality of a quenched ribbon is the viscosity of the molten metal before a quenched ribbon is produced from the molten metal. The viscosity of the molten metal varies with the temperature of the molten metal.
Accordingly, one can conceive of measuring the viscosity and the temperature of a molten metal of an alloy and controlling the quality of a quenched ribbon to be produced on the basis of the measurement results. However, it is difficult to directly measure the viscosity and the temperature of a high-temperature molten metal of an alloy.
Here, Japanese Patent Application Publication No. 2003-320442 discloses a quenched alloy manufacturing method in which a molten metal of an alloy is brought into contact with a rotating cooling roll to thereby quench the alloy and obtain an alloy containing a crystal phase. More specifically, JP 2003-320442 A discloses a quenched alloy manufacturing method including the steps of: preparing a molten metal of an alloy by heating an alloy; feeding the molten metal of the alloy onto the cooling roll; measuring the alloy temperature by detecting infrared light radiated by the alloy in motion in the solidification process of the molten metal of the alloy; and adjusting the quenching conditions on the basis of the alloy temperature.
Japanese Patent Application Publication No. 1-153938 and Japanese Patent Application Publication No. 2-45730 disclose slurry viscosity on-line measurement devices. Specifically, the devices are configured to measure the length of a continuous part of slurry released from a nozzle and a pressure under which the slurry is jetted out of the nozzle, and estimate the viscosity of the slurry from relations among the nozzle diameter, the length of the continuous part of the slurry, and the jet pressure of the slurry that are obtained in advance.
In the quenched alloy manufacturing method disclosed in JP 2003-320442 A, the alloy temperature is measured by detecting the infrared light radiated from the alloy in motion. However, the method of detecting the infrared light radiated from the alloy with an infrared thermometer may lead to a significant error between the actual alloy temperature and the measured temperature.
On the other hand, the viscosity measurement devices disclosed in JP 1-153938 A and JP 2-45730 A are claimed to be capable of measuring the viscosity of slurry with high accuracy and repeatability in any atmosphere, without being influenced by the temperature and the humidity of the atmosphere. However, this technique takes no account of the relation between the viscosity and the temperature of the slurry, and is confined to high-accuracy measurement of viscosity. That is, a temperature of the molten metal that is difficult to directly measure cannot be accurately specified by this technique.
In view of the close relation between the viscosity and the temperature of a molten metal of an alloy and the difficulty of directly measuring the viscosity and the temperature of a molten metal, the present inventors have devised a technique that can control the viscosity of a molten metal indirectly and accurately by specifying the melt temperature from another parameter and controlling the specified molten metal temperature so as to be within a proper temperature range, and can thereby manufacture quenched ribbons of excellent quality.